It is known in the art that the efficiency of a vapour/liquid contact column is dependent on a number of factors including distribution of the liquid within the column. Optimum column performance may be achieved if liquid distribution in the column is uniform over the area of horizontal cross-section throughout the length of the column. This condition may be referred to as “steady state”. Even small maldistributions of liquid can have a significant and adverse effect on column efficiency.
Steady state operation of a vapour/liquid contact column is possible when the column is stationary and aligned vertically (see FIG. 1). If the column is moved, for example, tilted out of vertical alignment, liquid distribution within the column is adversely affected (see FIG. 2). Tilting a vapour/liquid contact column from the vertical by as little as 1° can significantly reduce column efficiency.
In the case of land-based columns, such movement may be observed when columns (particularly tall, thin columns) sway due to high winds. However, movement is more apparent in the case of columns used offshore on floating platforms which are subject to “sea motion” where the rolling and pitching motion of the platform continually tilts the column out of vertical alignment (see FIG. 3). Under such conditions, the average inclination of the column is usually between 1° and 3° from the vertical but the column may often be inclined by as much as 10°. Large ships typically roll ±2° from the vertical and the time taken for one roll cycle is about 15 s. De Bussy (AIChE; March 2000) discusses of the type of problems encountered when using vapour/liquid contact columns offshore.
Oil and natural gas are both valuable natural sources of hydrocarbons. However, after extraction from the Earth, both oil and natural gas need to be processed into more useful products in order to maximise their usefulness. For example, oil may be refined into a variety of hydrocarbon products and natural gas may be converted into methanol or into Fischer-Tropsch hydrocarbon liquids, such as LPG, diesel and gasoline. The required processing facilities are ideally located at or near the sites of extraction as then it is not necessary to incur the substantial cost of transporting the oil and natural gas from the extraction facility to the processing facility. Such location does not usually present any problems in cases where the reserves of the natural products are on land. However, this is not the case where the oil and natural gas reserves are found offshore.
Oil and natural gas are usually transported between offshore extraction facilities and land-based processing facilities either by tanker or by pipelines laid on the sea bed. The associated capital and operating costs of such transportation methods are very high. For this reason, there is much interest in developing floating processing facilities for use offshore. Not only do such facilities have the benefit of significantly reducing the overall costs of producing useful products from these resources, but also the floating facilities themselves may be readily redeployed to other sites.
Vapour/liquid contact columns are usually packed with structured packing to increase the degree of contact between the ascending vapour and the descending liquid. EP-A-1036590 (Sunder et al; 2000) discloses corrugated structured packing suitable for a cryogenic air separation column.
U.S. Pat. No. 5,984,282 (Armstrong et al; 1999) discloses a vapour/liquid contact column for use offshore. The characterising feature of this column is the specific arrangements of sections of structured packing which reduce the adverse effects of sea motion on column efficiency.
U.S. Pat. No. 6,397,630 (Fraysse et al; 2002) discloses an offshore floating structure comprising a cryogenic air distillation column. The characterising feature of the floating structure is that the column is packed with cross-corrugated structured packing having a specific structure designed to negate the effects of sea motion on column efficiency.
Vapour/liquid contact columns are used for a variety of purposes in oil and natural gas processing. For example, oil may be fractionated in thermal distillation columns. In addition, sea water may be deoxygenated using vapour/liquid contact columns prior to injection into an oil reserve for pressure maintenance. Further, air may be separated in a cryogenic distillation column system to produce oxygen which may then be used for a number of purposes including converting natural gas into synthesis gas. The synthesis gas may be converted using the Fischer-Tropsch process to liquid hydrocarbons, such as LPG, diesel and gasoline, or the synthesis gas may be converted to methanol. Vapour/liquid contact columns are also used for separation of Fischer-Tropsch reaction products, for liquefied petroleum gas (“LPG”) recovery, for feed gas treatment on natural gas liquefaction plants and for gas drying operations. The present invention has application in each of these examples of vapour/liquid contact columns.
The results of a number of investigations into the effects of tilt and motion on vapour/liquid contact column performance have been published. For example, Tanner et al (Trans. I. Chem. E.; 1996; E.74.A; 177-182) studied the effects of tilt and motion on liquid distribution in a column packed with polypropylene Pall rings. In addition, Hoerner et al (CEP; November 1982; 47-52) studied the effects of tilt and motion on the separation of a mixture of methylcyclohexane and toluene using a 5 m column packed with regular packing.
Tilt and motion effects on a water deaeration column were studied by Tanner et al (Proc. Distillation and Absorption; 1992; I. Chem. E. Symp. Series No. 128; B.111-B.118). In this study, the column was packed up to a packed height of 2.45 m with six sections of structured packing having a surface area of 250 m2/m3.
Tilt and motion effects on liquid distribution with a vapour/liquid contact column have been studied by Waldie et al (AIChE Distillation Meeting; April 2004). In this study, the column was packed to a packed height of up to 4 m with 20 layers of structured packing having a surface area of 500 m2/m3.
After carrying out further studies into tilt and motion effects on liquid distribution within vapour/liquid contact columns, the Inventors of the present invention have discovered that the height of a packing section and the specific surface area of the structured packing are important factors for reducing liquid maldistribution in a column in motion. The above-mentioned publications do not disclose the importance of either of these factors to liquid distribution within such a column in motion.
It is an objective of preferred embodiments of the present invention to reduce the level of liquid maldistribution usually observed when a vapour/liquid contact column is subjected to motion and thereby to improve the efficiency of the column.